1. Field of the Invention
The invention provides a method of supporting a growing single crystal in the region of a conical section of the single crystal via a supporting body during crystallization of the single crystal according to the FZ method.
2. Description of the Related Art
Production of a single crystal according to the FZ method (floating zone method), also known as zone melting, comprises growing a single crystal in an upright orientation at a phase boundary disposed at the upper end of the growing single crystal between a melt and the growing single crystal. Lowering the single crystal causes material of the melt to crystallize thus increasing the volume of the single crystal. The loss of melt arising as a result is compensated for by gradually melting a solid feed, for example a feed rod. Also known are methods where granular material is melted instead of a feed rod.
The growing single crystal is obtained by contacting a comparatively thin seed crystal with molten material of the feed. A thin neck is initially generated to achieve dislocation-free crystal growth. On the thin neck, the single crystal grows initially to form a conical section of increasing diameter and subsequently to form a cylindrical section of constant diameter. To ensure the most uniform possible crystal growth and the most uniform possible distribution of dopants the growing single crystal is rotated about its longitudinal axis. If during rotation, the sense of rotation is periodically reversed according to a particular pattern, this is referred to as alternating rotation.
The surface tension keeps the melt in position at the upper end of the growing single crystal. The melt exhibits corresponding sensitivity to perturbations, in particular to vibration of the growing single crystal. It is imperative that vibration which causes spillover of the melt and/or results in termination of the crystal growth of the single crystal be avoided. The risk of this occurring increases with increasing diameter and weight of the single crystal to be produced.
Measures intended to protect the growing single crystal from such perturbations were already proposed early in the development of the FZ method. DE 2652199 describes a method that provides for application of a cylindrical length of glass tube as a supporting body against the conical section of the growing single crystal. That method is no longer sufficient for producing the single crystals of markedly greater weight that are now customary. The glass body is no longer entirely able to cope with the mechanical stress and may break apart due to the supporting forces being exerted upon it.
DE 2455173 proposes supporting the end of a crystal rod using a multipiece funnel sleeve filled with glass spheres as stabilizing means. Such an apparatus no longer fulfils the requirements either, as the apparatus cannot achieve sufficient bracing of the growing single crystal. However, such bracing is necessary to absorb the forces that arise during alternating rotation.
EP 0 288 639 A1 employs a ring of a hard ceramic material or of a hard metal to support the single crystal.
The inventors of the present invention have found that a hard ring too has disadvantages. Disposed upon the surface of a growing single crystal are crystal ridges. Crystal ridges are elevations extending in the direction of growth of the single crystal which protrude from the lateral surface of the conical section and/or from the lateral surface of the cylindrical section of the growing single crystal. Application of the hard ring to the growing single crystal causes reaction forces to be transmitted to the hard ring particularly via the protruding crystal ridges. This can exert so much pressure onto the crystal ridges that the ridges collapse at the affected areas. The shock that follows this collapse and acts on the growing single crystal is a perturbation which causes the melt to vibrate and can cause spillover thereof.